The invention relates generally to x-ray tubes, and more particularly to structures and methods of assembly and operation for a hydrodynamic bearing utilized in an x-ray tube.
X-ray systems may include an x-ray tube, a detector, and a support structure for the x-ray tube and the detector. In operation, an imaging table, on which an object is positioned, may be located between the x-ray tube and the detector. The x-ray tube typically emits radiation, such as x-rays, toward the object. The radiation passes through the object on the imaging table and impinges on the detector. As radiation passes through the object, internal structures of the object cause spatial variances in the radiation received at the detector. The detector then emits data received, and the system translates the radiation variances into an image, which may be used to evaluate the internal structure of the object. The object may include, but is not limited to, a patient in a medical imaging procedure and an inanimate object as in, for instance, a package in an x-ray scanner or computed tomography (CT) package scanner.
X-ray tubes include a cathode and an anode located within a high-vacuum environment. In many configurations, the anode structure is supported by a hydrodynamic or liquid metal bearing structure, e.g., a spiral groove bearing (SGB) structure, formed with a support shaft disposed within a sleeve or shell to which the anode is attached and that rotates around the support shaft. The spiral groove bearing structure also includes spiral or helical grooves on various surfaces of the sleeve or shell that serve to take up the radial and axial forces acting on the sleeve as it rotates around the support shaft.
Typically, an induction motor is employed to rotate the anode, the induction motor having a cylindrical rotor built into an axle formed at least partially of the sleeve that supports the anode target and an iron stator structure with copper windings that surrounds an elongated neck of the x-ray tube. The rotor of the rotating anode assembly is driven by the stator. The x-ray tube cathode provides a focused electron beam that is accelerated across an anode-to-cathode vacuum gap and produces x-rays upon impact with the anode. Because of the high temperatures generated when the electron beam strikes the target, it is necessary to rotate the anode assembly at high rotational speed. This places stringent demands on the bearings and the material forming the anode structure, i.e., the anode target and the sleeve and shaft assembly supporting the target.
Advantages of hydrodynamic or liquid metal bearings such as spiral groove bearings in x-ray tubes include a high load capability and a high heat transfer capability due to an increased amount of contact area. Other advantages include low acoustic noise operation as is commonly understood in the art. Gallium, indium, or tin alloys are typically used as the liquid metal in the hydrodynamic bearing structure, as they tend to be liquid at room temperature and have adequately low vapor pressure, at operating temperatures, to meet the rigorous high vacuum requirements of an x-ray tube.
However, hydrodynamic bearings with liquid metal working fluids typically contain one or more journal bearings and one or more thrust bearings. For hydrodynamic bearings, journal bearings are typically concentric cylinders with a small radial gap for working fluid whereas thrust bearings are typically flat parallel flanges with a small axial gap for working fluid. A common issue with these types of bearings is they have zero load capacity when not rotating or when the speed of rotation is low enough for the liquid metal to be unable to support the rotating component of the bearing assembly. As such, when the hydrodynamic bearing slows below a certain minimum rotational speed at which the liquid metal can support the rotating component, e.g., the sleeve, the sleeve tends to land on/contact the non-rotating component at some non-zero rotational speed. This surface to surface relative motion between the rotating and non-rotating bearing components causes wear and reduces life and performance of the bearing assembly. The rotating sleeve conventionally lands upon the non-rotating component/shaft of the bearing assembly (the journal bearing) which inherently has small contact surface areas (nearly line contacts), thereby creating a large landing stress. Landing of hydrodynamic bearings upon journal bearings is a major source of wear in the hydrodynamic bearing assembly and is a significant failure mode for x-ray tubes. Undesirable wear is also associated with hydrodynamic bearing take-off, that is, driving the rotating component from rest to the required operational frequency. During take-off, the rotating component briefly does not have sufficient relative speed to the stationary component for the working fluid to support the required load, thus bearing surfaces rub and cause wear as described previously.
Therefore, it is desirable to develop a structure and method for the operation of a bearing assembly or structure for an x-ray tube that reduces the stresses on the rotating component of the bearing assembly to significantly decrease wear resulting from the landing or take-off of the rotating bearing component on the non-rotating component, resulting in increased useful life for the bearing assembly.